1. Field of the Invention
This invention relates to the sealing of container closures, particularly of those closures intended to be opened by the users.
2. Description of the Prior Art
Spouted, gable-topped or flat-topped cartons made from paperboard coated on both faces with thermoplastics are conventional. It is well known to seal the top and bottom closures thereof, of which the top incorporates an openable spout, by means of heating thermoplastic surface portions with hot air to a tacky consistency and then, by use of a pair of jaws, pressing the tacky portions together while they cool, thereby to form a fin. However, with the fin including part of the spout, there is a conflict between the need to ensure that the fin is liquid-tight and thus well-sealed and the consumer requirement that the spout should be easy to open. As a step towards the resolution of this conflict, it is also well known to provide abhesive substance over selected face-to-face surface portions of the fin at the spout to deter sealing, to facilitate opening of the spout.
Such hot air heating tends to be non-uniform and inaccurate and involves two process stations. It is known from U.S. Pat. No. 3,309,841 and European patent specification No. 0028941A to use nozzles in the form of lines of perforations or slots to direct the hot air towards those surface portions to be rendered tacky in order to try to minimize heating of those surface portions which are not to be sealed together, particularly of selected surface portions of the spout. In the disclosure of the European patent specification, the jaws are provided with respective shallow elongate recess to relieve the pressure of the jaws on abhesive-coated surface portions of the spout at the fin. However, even this system still suffers from inaccuracy and non-uniformity, resulting in poor sealing at the fin and/or difficulty of opening of the spout.
For these reasons ultrasonic welding employing an ultrasonic horn and an anvil has increasingly been used recently; it can be both accurate and uniform.
U.S. Pat. No. 3,912,576 discloses carton sealing apparatus for sealing a rectangular bottom closure of a thermoplastic coated paperboard carton by ultrasonic vibrations. The apparatus includes an anvil and an ultrasonic vibrating tool. The mandrel is received in the carton with the closure to be sealed folded against the smooth end surface thereof. The ultrasonic vibrating tool engages the closure on the opposite side thereof from the mandrel and, then energized, generates heat sufficient to cause the thermoplastic coating of the paperboard to flow and weld the layers of the closure together. Ribs project from the end surface of the vibrating tool to provide a sealing pattern of "bow-tie" configuration, the ribs having various heights with respect to the end surface of the vibrating tool to accommodate changes in the number of layers of paperboard at the different areas on the closure. Somewhat similar apparatus are disclosed in U.S. Pat. Nos. 3,971,300 and 4,011,800.
U.S. Pat. No. 3,956,975 discloses a method and an apparatus for making a spouted gable-topped carton from a flat blank, in which a corner seam of the carton, the flat bottom closure and the gable-top closure are sealed at separate stations all by means of ultrasonic welding. The sealing of the fin of the gable-top closure is performed by a retractable, concentrating vibrating horn to one side of the path of advance of the filled carton and a retractable anvil to the opposite side of the path. To cope with a change in the fin from an upper two-ply part to a lower four-ply part, either of two alternative constructions of ultrasonic welding device are employed. In the first, the anvil is a single piece anvil having a configuration to accommodate the changing contour from two-ply to four-ply at its side of the fin and the horn is a single piece horn having a configuration to accommodate the changing contour from two-ply to four-ply at its side of the fin. In the second a separate horn and anvil are utilized to weld the two-ply part and a separate horn and anvil are utilized to weld the four-ply part. In both cases, the welding extends as bands of constant width from one end of the fin to the other.
Unfortunately, with this welding arragement, the spout is difficult to open if the fin is well sealed. Somewhat similar apparatus are disclosed in U.S. Pat. Nos. 4,072,089 and 4,159,220.